Tissue may be destroyed, ablated, or otherwise treated using thermal energy during various therapeutic procedures. Many forms of thermal energy may be imparted to tissue, such as radio frequency electrical energy, microwave electromagnetic energy, laser energy, acoustic energy, or thermal conduction. In particular, radio frequency ablation (RFA) may be used to treat patients with tissue anomalies, such as liver anomalies and many primary cancers, such as cancers of the stomach, bowel, pancreas, kidney and lung. RFA treatment involves destroying undesirable cells by generating heat through agitation caused by the application of alternating electrical current (radio frequency energy) through the tissue.
Various RF ablation devices have been suggested for this purpose. For example, U.S. Pat. No. 5,855,576 describes an ablation apparatus that includes a plurality of electrode tines deployable from a cannula. Each of the tines includes a proximal end that is coupled to a generator, and a distal end that may project from a distal end of the cannula. The tines are arranged in an array with the distal ends located generally radially and uniformly spaced apart from the distal end of the cannula. The tines may be energized in a bipolar mode (i.e., current flows between closely spaced electrode tines) or a monopolar mode (i.e., current flows between one or more electrode tines and a larger, remotely located common electrode) to heat and necrose tissue within a precisely defined volumetric region of target tissue. To assure that the target tissue is adequately treated and/or to limit damaging adjacent healthy tissues, the array of tines may be arranged uniformly, e.g., substantially evenly and symmetrically spaced-apart so that heat is generated uniformly within the desired target tissue volume.
When using the above described devices in percutaneous interventions, the cannula is generally inserted through a patient's skin, and the tines are deployed out of the distal end of the cannula to penetrate target tissue. Particularly, the tines are deployed such that the distal ends of the tines initially exit from a distal opening at the cannula. As the tines are further deployed, the distal ends of the tines evert radially away from an axis of the cannula, and then back towards a proximal end of the cannula (so that they face substantially in the proximal direction when fully deployed). As such, the tines/electrodes of the above described device each has a profile that resembles a parabola after the electrodes are deployed. The tines are then energized to ablate the target tissue.
Sometimes, a vessel or sensitive tissue may be located at or adjacent to a distal region of a target tissue that is to be ablated. In such cases, use of the above described devices may perforate the blood vessel and/or injure the sensitive tissue. Particularly, deploying the electrodes at the distal end of the cannula such that the deployed electrodes evert proximally towards a proximal end increases the risk that the electrodes will injure the blood vessel or the sensitive tissue as the electrodes are being deployed.
In addition, when using heat to kill tissue at a target site, the effective rate of tissue ablation is highly dependent on how much of the target tissue is heated to a therapeutic level. In certain situations, complete ablation of target tissue that is adjacent a vessel may be difficult or impossible to perform, since significant bloodflow may draw the produced heat away from the vessel wall, resulting in incomplete necrosis of the tissue surrounding the vessel. This phenomenon, which causes the tissue with greater blood flow to be heated less, and the tissue with lesser blood flow to be heated more, is known as the “heat sink” effect. It is believed that the heat sink effect is more pronounced for ablation of tissue adjacent large vessels that are more than 3 millimeters (mm) in diameter. Due to the increased vascularity of the liver, the heat sink effect may cause recurrence of liver tumors after a radio frequency ablation.
Thus, there remains a need to provide for improved ablation devices.